Research On The Mechanism Of Cracks In Cement-based Materials Repaired By Microorganism Under Marine Environment

With the construction and service of offshore infrastructure,more and more cracks appear in the concrete structures,which not only affects the durability and service life of the concrete structures,but also endangers the safety of personnel in severe cases.To prevent or repair cracks in offshore concrete structures effectively,microbial induced carbonate precipitation（MICP）technology is adopted.The technology is a safe and environmentally-friendly technology that can effectively repair cracks,which not only reduces the impact of harmful medias on the durability of concrete structures in the marine environment,but also decreases the maintenance cost and meets the repair requirements of some micro-cracks in complex structures.At present,MICP technology has been widely used in the self-healing of cracks in concrete structures under ordinary environment,but the research on self-healing of cracks in marine environment is not sufficient.This article compared the growth and mineralization characteristics between Bacillus subtilis（BS）and Bacillus pasteurii（BP）in the marine environment,then mixed them into marine cement to prepare microbial-based marine mortars,and comprehensively studied the self-healing behavior and mechanism of the cracks for the mortars in the atmospheric zone,the immersion zone and the tide zone.The results are as follows:（1）It can be known by testing the OD₆₀₀ of the bacterial solution and analyzing the characteristics of the sediment,seawater inhibited the growth and mineralization of the BP but promoted those of the BS.When the culture medium concentration in the bacterial solution was 6 g/L,the initial p H=9,and the urea concentration was 2%,the BS-induced calcium carbonate deposition reached the highest values of 5.6 g/L,19.6 g/L,and 22.2 g/L,which was 7 times,15.2 times and 9.3 times that of BP,respectively.In addition,adding Ca Cl₂ to the bacterial solution,as its concentration increases,calcium carbonate deposition induced by both BP and BS increases.And the mineralization ability of BS was higher than that of BP,but after its concentration≧1 mol/L,the amount of calcium carbonate induced by the BS tended to be stable.（2）Testing by means of compressive strength,chloride ion content,drying shrinkage,mercury intrusion method（MIP）,etc.,the marine mortar had good mechanical properties,high resistance to chloride ion penetration and low drying shrinkage.Among them,the compressive strength of the 56-day mortar was higher than 97%of ordinary mortar;the chloride ion diffusion coefficient and drying shrinkage of the 90-day mortar were lower than53.6%and 53.4%of ordinary mortar,respectively.Which was attributed to its high content of ettringite（AFt）and calcium silicate hydrate（C-S-H）,low content of calcium hydroxide（CH）,small CH crystal size and dense pore structure.（3）Testing by means of flexural strength,compressive strength,electric flux,water absorption,MIP,etc.,although the porosity and mechanical properties of the microbial-based marine mortar（Bacillus subtilis-based marine mortar（BSM）and Bacillus pasteurii-based marine mortar（BPM））were higher and lower than those of ordinary mortar（OM）in the early stage of hydration respectively,the pore structures were denser,mechanical properties were better,capillary water absorption and chloride migration were lower in the later stage of hydration.（4）The self-healing behavior and mechanism of microbial-based marine mortar cracks in three marine environments（atmospheric,immersion and tide zones）are revealed,as follows:In the atmospheric zone,because the content of H₂O in the air was not enough to activate the microorganisms,the cracks of the BPM and BSM had no obvious self-healing phenomenon like that of OM.The self-healing was mainly based on the autogenous healing;In the immersion zone,although there was sufficient H₂O in the mortar cracks,a small amount of CO₂ and O₂ in the seawater provide limited support for the mineralization of microorganisms.Therefore,it was found that the cracks of BSM showed slight self-healing,and there was Ca CO₃ precipitation on its crack surfaces,while the obvious self-healing phenomenon were not observed in the cracks of OM and BPM.The self-healing products such as CH crystals inside the cracks were distributed in the form of loose particle piles;In the tide zone,a large amount of H₂O,CO₂ and O₂ provided favorable conditions for the self-healing of mortar cracks.Therefore,OM,BPM and BSM cracks all had obvious self-healing phenomenon,and the degree of self-healing was BSM>BPM>OM,among them,the self-healing of OM cracks was mainly based on autogenous healing and the chemical factors such as Mg²⁺in seawater reacts with OH-in the mortar to form Mg（OH）₂.The cracks of BPM and BSM are mainly microbial-induced mineralization besides the original self-healing and chemical factors.The surfaces of its cracks had Ca CO₃ precipitation,which effectively prevented the outside air and seawater from penetrating into the cracks.Therefore,the CH crystal in the cracks was not consumed in large quantities and retained a high level of its content.In addition,compared with the air and immersion zones,the tide zone was more conducive to the self-healing of mortar cracks.The microbial-based marine mortars are expected to serve the offshore infrastructure such as cross-sea bridges,port terminals,offshore oil platforms,and especially the South Island Reef Project.